Motor vehicle designers continually strive to create vehicles which have lower emissions of noxious and greenhouse gases than vehicles currently in use. One means of reducing vehicular emissions is to utilize alternative fuels. Commonly used fuels, such as gasoline and diesel fuel, are mixtures of complex hydrocarbons which may also contain unwanted chemicals, such as sulfur. One form of alternative fuel, for non-limiting example, is liquefied petroleum gas (LPG). LPG is primarily composed of propane, a three carbon hydrocarbon, and butane, a four carbon hydrocarbon. These hydrocarbons have a lower carbon to hydrogen ratio than gasoline or diesel fuel. Because the carbon to hydrogen ratio is lower, less carbon dioxide is produced in the burning of LPG than in the burning of gasoline or diesel fuel. The longer chain hydrocarbons of gasoline and diesel fuel are also more likely to produce unwanted particulate emissions in the exhaust gas. Relative to LPG, gasoline and diesel fuel do have two advantages, namely: (i) both gasoline and diesel fuel are liquids at standard temperature and pressure (STP), whereas under typical ambient operating conditions LPG must be stored in a pressure vessel to be kept in a liquefied state; and, (ii) gasoline and diesel fuel produce more energy per unit volume of fuel as compared to LPG, even when LPG is in a liquid state.
System functionality of prior art fuel systems utilizing alternative fuels is generally as follows. Fuel is supplied to a fuel consumer via a vaporizer and pressure regulator (vaporizer-regulator). The fuel comes from a fuel tank which stores the fuel at, or near, its vapor pressure. The fuel supplied to the vaporizer-regulator consists of liquid fuel, fuel vapor or a mixture of liquid fuel and fuel vapor. Under normal operating conditions, the fuel supplied to the vaporizer-regulator consists predominantly of liquid fuel, but may also include some fuel vapor. Under unusual operating conditions (e.g., when the fuel tank is nearly empty), the fuel supplied to the vaporizer-regulator may consist mostly, or entirely, of fuel vapor. Under normal operating conditions, the fuel supplied to the vaporizer-regulator meets a minimum fuel feed pressure requirement at the vaporizer-pressure regulator. This minimum fuel feed pressure is the pressure required to ensure that the vaporizer-regulator will function correctly and that the target fuel feed pressure downstream of the pressure regulator can be maintained to supply the fuel demand of the fuel consumer. For some prior art fuel system embodiments, the minimum fuel feed pressure requirement may vary, depending on factors such as the backpressure at the fuel consumer, the flow rate of fuel to the fuel consumer, the density of the fuel supplied to the fuel consumer, or the rate of energy flow out of the fuel consumer (e.g., if the fuel consumer is an internal combustion engine, the minimum fuel feed pressure requirement may depend on the inlet manifold pressure, fuel flow rate, fuel density, driveshaft torque, power output and/or rotational speed). Some prior art embodiments may be fitted with a fuel pump to increase the fuel feed pressure in circumstances where the vapor pressure of the fuel in the fuel tank is not large enough to meet the minimum fuel feed pressure requirement at the vaporizer-regulator.
In general, prior art fuel systems utilizing alternative fuels must address the following functionality requirements: 1) meet the minimum fuel feed pressure requirement at the vaporizer-regulator (wherein, this minimum fuel feed pressure requirement may vary, depending on various factors, such as the backpressure at the fuel consumer, the flow rate of fuel to the fuel consumer, the density of fuel supplied to the fuel consumer, or the rate of energy flow out of the fuel consumer); and 2) meet minimized operation and/or duty cycle of fuel pumping system (wherein, the objective is to reduce noise generated by fuel pumping and/or reduce electrical energy consumption and/or increase service life of the fuel pumping system).
FIGS. 1A and 1B depict examples of typical prior art LPG fuel systems 10, 10′, wherein identical numbers are used for referencing identical parts, and wherein FIG. 1B is truncated for brevity. These prior art LPG fuel systems are of the gaseous-phase manifold-injection type, as for example utilized by an internal combustion engine of, for example, a motor vehicle, wherein the system meters LPG fuel into the inlet manifold of the engine in the gaseous phase.
A pressurized fuel tank (or vessel) 12 holds LPG fuel contents 15 in liquid phase 15′ and vapor phase 15″. The fuel tank 12 is equipped with a tank pressure relief valve 13, and may be equipped with a temperature sensor 14 and a pressure sensor 16. The LPG fuel 15 of the fuel tank 12 may be subject to external heat 17, as for example coming from the motor vehicle exhaust system outside the fuel tank, and in the example of FIG. 1, to heat 19 from components within the fuel tank, as for example due to operation of the fuel pump 26, wherein all of these sources of heat increase the temperature of the LPG fuel 15, thereby increasing the vapor pressure inside the fuel tank.
Contained within the fuel tank 12 are, in the example of FIG. 1A, components that make up the fuel line pumping system 18, and in the example of FIG. 1B, simply a filter 24 at a lead end of the fuel line 22. In both the examples of FIGS. 1A and 1B, liquid phase fuel 15′ of the LPG fuel 15 is extracted via the fuel line 22 after first passing through the filter 24 in order to prevent debris from entering the fuel line.
In the example of FIG. 1A, the filter 24 connects to the fuel pump 26. The fuel pump 26 is typically engaged to boost fuel feed pressure when the pressure inside the fuel tank 12 is below a predetermined level, as for example a pressure of 3 bar (absolute). After passing through the fuel pump 26 the fuel passes through a check valve 25 and then through a filter 28. A fuel pressure regulator 20 controls the pressure differential across the pump 26 and the fuel filter 28 so that a desired fuel pressure differential is maintained, as for example, a pressure differential of about 2.5 bar. Fuel filter 24 is located upstream of fuel pump 26 and has low pressure resistance so as to minimize cavitation inside the fuel pump. Fuel filter 24 may take the form of a strainer, while fuel filter 28, by contrast, is a finer filter, designed to prevent debris passing into the fuel system components downstream of fuel pump 26 and its check valve 25. The pressure drop across fuel filter 28 can be larger than the pressure drop across fuel filter 24 because fuel filter 28 is located downstream of fuel pump 26, and, therefore, fuel pump cavitation will not occur due to the pressure drop across fuel filter 28.
In the examples of both FIGS. 1A and 1B, the fuel line 22 connects to a valve set 36 which is ordinarily mounted on the wall of the fuel tank. The valve set 36 typically includes: a flow control valve 30 for preventing excessive flow in the event of accidental rupture of the downstream fuel line; a service valve 32 which is always open during the normal operations of the vehicle, but when the vehicle is being serviced, a technician can turn off the service valve 32 manually to thereby isolate the fuel tank 12 and related structures from the rest of the fuel line of the vehicle; and an automatic fuel shut-off solenoid valve 34 which is typically controlled by a controller or electronic control module (ECM), not shown, wherein the controller coordinates information from one or more sensors, not shown, in order to detect conditions that require a shut down of the fuel flow through the fuel line 22. A typical example of such a condition is when the driver switches engine ignition ‘off’.
The fuel line 22 then connects to another automatic fuel shut off solenoid valve 40, wherein the controller thereof, also not shown, coordinates information from one or more sensors, not shown, in order to detect conditions that require a shut down of the fuel flow through the fuel line 22. After passing through the shut off solenoid valve 40, the fuel passes into a vaporizer-regulator 38. The vaporizer 44 thereof typically includes a heat exchange circuit 46 where heat is extracted from the engine cooling system 46a for the purpose of converting fuel which is in a liquid or vapor phase to a superheated gaseous phase. The fuel may also be heated by an electric heater 48a, wherein the heat exchanger which exchanges heat with the engine coolant is often the same as the heat exchanger which exchanges heat with the electric heater. This device provides the additional heat necessary to fully vaporize the fuel to a superheated gaseous state during engine warm-up, when the engine cooling system is still relatively cold. The gaseous fuel then passes into a pressure regulator 50 of the vaporizer-regulator 38, wherein the pressure regulator controls the pressure of the fuel in the fuel rail 54 and is typically referenced to inlet manifold pressure; for example, the pressure regulator 50 may maintain a pressure differential of 1 bar between inlet manifold pressure and fuel rail pressure. The final component of the vaporizer-regulator 38, or as a stand-alone component, is a pressure relief valve 52 which prevents the pressure of the gas in the fuel rail exceeding a predetermined level, typically opening at between 2 and 3 bar, which could arise under certain operating conditions, such as when there is no flow of fuel out of the fuel rail. If the pressure of the gaseous fuel in the fuel rail exceeds this predetermined level, then the pressure relief valve 52 opens and releases excess fuel from the downstream side of the pressure regulator 50 to a lower-pressure location such as the engine inlet manifold, not shown. It should be noted that the pressure relief valve 52 is separate and different from the aforementioned pressure relief safety valve 13 fitted to the fuel tank 12, which will typically open at between 26 and 28 bar.
Once the gaseous fuel passes the pressure relief valve 52, it enters a fuel rail 54 which serves to distribute the fuel to the cylinders 64a-64f of the engine 70, wherein the engine, which serves in the capacity of a fuel consumer in this application, may have any number of cylinders, six being shown merely by way of example, and wherein each cylinder possesses a fuel injector 56a-56f which is typically controlled by a controller or ECM, not shown. The fuel rail 54 may also be provided with a temperature sensor 60 and a pressure sensor 62 so that the data therefrom may be used by the engine system controller, not shown.
FIG. 1C shows an example of an algorithm 80 indicative of typical steps for the operation of the prior art LPG gaseous-phase manifold-injection fuel system 10 of FIG. 1A.
At Block 82, the system is initialized when the fuel consumer is turned on, e.g., the ignition switch starts the engine 70. The algorithm then advances to Block 84, whereat data may be obtained by sensors, calculated or obtained from a look-up table, including the fuel feed pressure at or upstream of the vaporizer-regulator 38. The algorithm then advances to Decision Block 86, whereat inquiry is made as to whether fuel pumping is required in order to supply a minimum fuel feed pressure to the vaporizer-regulator which satisfies the fuel demand of the fuel consumer. If the answer to the inquiry is yes, then at Block 88 fuel pumping is activated, or if already activated then fuel pumping remains activated, and the algorithm returns for Block 84. However, if the answer to the inquiry at Decision Block 86 is no, then the algorithm proceeds to Block 90, whereat fuel pumping remains deactivated, or if already activated then fuel pumping is deactivated. The algorithm then returns to Block 84.
One issue which can be encountered with prior art LPG fuel systems, as well as other pressurized fuel systems in general, concerns refilling (or refueling) when the pressure within the fuel tank is too high relative to the pressure at the nozzle of the bowser (or refilling station) to permit reasonably rapid refilling, or indeed, too high to permit any refilling (a no-fill situation). For rapid refilling to occur, the pressure of the bowser nozzle should be well in excess of the fuel vapor pressure within the fuel tank. As this pressure differential becomes smaller, so too does the rate of refilling become smaller. Thus, it is desirable for the vapor pressure in the fuel tank to be at or below a predetermined vapor pressure threshold by which it is deemed that a desired rate of refilling will be provided, as per an anticipated pressure of the bowser nozzle. This concern is exacerbated for fuels having multiple components of varying volatility.
Another issue which can be encountered in prior art LPG fuel systems, as well as other pressurized fuel systems in general, concerns delivery of vapor to the fuel consumer thereof during cold episodes (as for example a cold engine start in cold weather), wherein the ability of the vaporizer to provide complete fuel vaporization may be less than required to achieve 100% fuel vaporization at a mass flow rate demanded of the fuel consumer, whereby the consumer could become starved of fuel.
Accordingly, some means of managing the temperature and/or the pressure of the fuel inside the fuel tank is desirable in order to limit the maximum vapor pressure inside the fuel tank with speed and ease of fuel tank refilling in mind, while also having some ability to extract vapor as an assist to the vaporizer.